1. Field of the Invention
This invention generally relates to a variable gain amplifier, and especially to a current-matching variable gain amplifier.
2. Description of Related Art
Following the rapid development in radio communication field, a lot of people have been engaged in the research of variable gain amplifier. Up to now, though many solutions have been introduced, there is still room for improvement.
FIG. 1 is a schematic circuit drawing of a variable gain amplifier according to an U.S. patent No. 2003/0181181. Wherein, Input is an input terminal, Output is an output terminal, and Inductor 202 is a load of the amplifier circuit. The amplifier utilizes a control signal 201 to turn on or turn off n-channel metal oxide semiconductor field effect transistor (i.e. NMOSFET, as NMOS transistor hereinafter) 211 and 212, for changing a current passing through the inductor 202, and switching between two gains. At a high gain mode, the control signal 201 would turn off the NMOS transistor 211 and turn on the NMOS transistor 212, such that the total current pass through the inductor 202, to achieve a high gain. At a low gain mode, the control signal 201 turns on the NMOS transistor 211 and turns off the NMOS transistor 212, such that a part of the current passes through the inductor 202, to achieve a low gain. The amplifier circuit is simpler than the previous amplifier. But the problem is, in order to achieve an accurate gain value, the sizes of the NMOS transistors 211 and 213 must be appropriately matched, and this problem is not easy to solve in the limitation of the transistor manufacturing process. In order to make the most of the current pass through the NMOS transistor 211 for achieving the low gain mode, the transistor size must be bigger, so that the power wastage at the high gain mode is higher and the noise characteristic is worse.
FIG. 2 is a schematic circuit drawing of a variable gain amplifier according to U.S. Pat. No. 6211737. Wherein, Input is an input terminal of the amplifier, and Output is an output terminal of the amplifier. Like the previous amplifier, this one utilizes a bypass method. That is, the amplifier utilizes the control signals 301 to 303 separately to turn on or turn off bipolar junction transistors (BJT transistor hereinafter) 311 to 313, so that branch circuits 321 to 323 either share or not share the current, for changing the current value passing through BJT transistor 310 and switching among different gains. The amplifier and the above mentioned amplifier has the same disadvantage. That is, the size of the transistors cannot be matched easily and the power wastage at high gain mode is higher.
Accordingly, a better and simply designed circuit is desired for further improving the technical problems in the prior art.